Milkymist One SMT/DIP Process Flow

Here collected info is came from visiting smt factory during producing Milkymist One RC1 pcba. Not only introductions about how Milkymist One trial run holds but also other SMT[1] & DIP (also short for Through-Hole[2]) equipment introductions.

Printing machine is to print solder paste on pcb through Screen-Printing[4] process. The pattern of solder paste layer designed during pcb rounting. Normally you don't need to specify a well-known brand solder paste to pcb maker, as long as request them to use a lead-free paste. But they all have famous brands' Solder Paste, for example;

Stencil is a material for the process of printing soldering paste of smt stage. There are two methods to produce stencil: One is made by gerber file, the other is made by pcb optical scanning.

by gerber file: the smt factory will request customer to get final gerber file which is exact
mapping with instant pcb produced from pcb maker not from customer's routing files. This approach
is more significant to mass production without much variable tolerances.

by optical scanning: the smt factory also can use the instant pcb sample to let stencil maker to do a scan to copy and produce it.

For Milkymist One RC1 we make this by file which is brought from New Heart Corporation. Thus is also this time RC1 pcb maker. For an EMS or SMT industry, they almost frequently build oriented-business with stencil makers like http://www.cksmt.com.tw or http://www.laser.com.tw/. In Taiwan for example the quotation about pcba assembly including outsourcing stencil made on smt factory side. The Milkymist One RC1's stencil is made by Chian Kang[5], it's a thickness with only 0.15mm. Thus the solder paste at least can be printed on pcb with 0.15mm thickness.

A number of Stencils storing in rack.

Milkymist One stencil.

The stencil file is somehow the same as the layer SOLDER PASTE in gerber files generated in PADS, OrCad or Altiem Designer tools. Sometimes for economic goals in mass production effectively, gerbers in PANEL structures rather than single gerber. Eg., 2 pieces, 4 pieces, or even more pieces in one panel. That's the reasons that must use FINAL SOLDER PASTE layer film/file produced by pcb maker.

Before going for Pick & Place process, there's a step that to feed parts into AI machine. A feeder doing this bridge between parts and artificial intelligence arm. To achieve this, customer needs to provide bom with indicated different packages so that the feeders will be used differently. See follows:

The reflow soldering process for Pb-free components is very similar to the conventional eutectic solder reflow process. However, there are some important differences that must be taken into consideration for Pb-free soldering as the soldering material used for Pb-free soldering is different and higher reflow temperatures are required. The all critical parts on even top or bottom side of boards, with a measured temperature which coincides with the curve of recommended temperature is necessary. The most concerned one is Spartan-6 fpga's Pb-Free Reflow Soldering Guidelines which is also called recommended IR-Reflow profile generally. Be careful of the peak temperature(body) is 250°C max. The IR/Convection is 60–150 seconds (60–90 seconds typical) during temperature maintained above 217°C. The peak temperature(ball/lead) is 245°C typically. That depends on solder paste, board size and components mixture. So far we are at prototype stage, a bit 1~2°C over 245°C can usually solder well. This sometimes only done by customer requested. The smt factory will never take a risk to do this unless you had have confidence on this or strongly request.

how to do this measurement?

Step1: SMT technician enter the value of temperature setting of each reflow room. All entered value may not equal to the actual value, It doesn't matter. An exact temperature profile will be plotted soon.

Step2: Using a thermal sensor. The above is an 6ch thermal sensors recorder.

Step6: Using Profile Reader to read measured temperature and plot their curve.

Step7: Plotted temperature profile. With this, you can compare it with chip's Pb-Free Reflow Soldering Guidelines. Or check their internal report.

Milkymist One boards are going to SMD IR Reflow machine.

Milkymist One boards are going out from SMD IR Reflow machine.

If the plotted temperature profile is not the same or meeting to chip manufacturer's recommended Pb-Free Reflow Soldering Guidelines, you need to ask smt technician to fine turn reflow machine settings to get a correct data. Thus do Step 1 and from Step 4 to Step 7 again.

how to make sure what kinds of shape look will have a good solderability?

Checking if solderability of bga is a really tough job seriously without accurate equipments. At least we need to know more from your design firstly. Before recognising the images of inspections, please check as your design geber file first: ball size layer which is called normally Paste Mask Top layer, Paste Mask Top with tracks(thus is Top copper layer) and Paste Mask Bottom layer.

I brought #2 by myself and went to X-Ray station there too late. They have already started taking inspections. Those pictures are brought back from Minbo supervisor. So the result of #2 will be added later.
Later I took X-Ray inspections for #2 pcba from another vendor, they used a DAGE brand XD6500. So the follows are the results from this machine. It costs USD 15 for first time customer. The official price is USD 91 in 1 hour to take no matter how many images you want.

Humans are a prime source of electrostatic discharge. As known a Human Body Model(HBM)[10]. A charged object readily transfers its charge to a person's conductive skin layer. Operators do test if wrist strip works well before working. Also pass through Air Shower before entering smt room in order to blow off dust.